Fill level measurements are applied in a wide variety of industries, e.g. in the manufacturing industry, in the chemical industry, and in the foods industry. The fill level measurements are used e.g. for control and/or regulation of manufacturing and/or materials treatment processes.
In fill level measurement technology, three types of fill level measurement are presently known. These three types differ according to the type of parameter which is recorded in the measurement. The different parameters can be divided into integral, differential, and discrete parameters.
An integral parameter is e.g. the pressure difference between a bottom and a top of the container. The pressure difference changes depending on the fill level. Another integral parameter is a fill-level dependent capacitance between an electrode protruding into the container and the container.
If an integral parameter is used for fill level measurement, then, however, the danger exists that material parameters, which enter into the fill level determination, e.g. a density of the liquid when the fill level is established using a pressure difference measurement, or a dielectric constant of the fill substance-when the fill level is determined using a capacitance measurement, either are not precisely known in advance, or can fluctuate.
A differential parameter, which can be used for determining fill level, is e.g. a material flow rate into the container and/or out of the container. This can be registered e.g. using flow rate measuring devices. With flow rate measuring devices, changes in fill level can be registered more precisely than with fill level measuring devices, given that in this instance, an occurring measuring error relates only to the flowing amount of material and not to the entire amount of the fill substance.
If the fill level is known at a starting point, then a fill level at a later point can be determined from the material flow rate by integration.
However, in the case of integration, systematic measuring errors can add up, such that in the long run, a growing uncertainty of the measurements will arise.
Additionally, the possibility exists to determine fill level using discrete parameters. The most significant discrete parameter for a continuous fill level measurement is the position of an upper surface of the fill substance. This position can be determined e.g. using fill level measuring devices operating according to the travel time principle. Signals, e.g. electromagnetic signals or ultrasonic signals, are transmitted to the surface, and their echo signals are received back. A travel time to the surface and back is determined, and from that, the position of the surface is determined.
Presently a wide variety of such methods are in use, e.g. the Pulse-Radar method, the Frequency Modulated Continuous Wave (FMCW) method, and Time-Domain-Reflectometry.
In the Pulse-Radar method e.g. short send-pulses, normally microwave pulses, are periodically transmitted by means of an antenna to the surface of a fill substance, and the echo signals reflected on the surface are received back after a separation-dependent travel time. An echo function representing the echo amplitudes as a function of the travel time is established. Each value of this echo function corresponds to the amplitude of an echo reflected at a specific separation from the antenna.
From the echo function, a usable echo is determined, which corresponds to the reflection of a send-pulse on the upper surface of the fill substance. It is normally assumed that the usable echo has a greater amplitude than the remaining echoes. In the case of a fixed propagation velocity of the send-pulses, the separation between the upper surface of the fill substance and the antenna is immediately obtainable from the travel time of the usable echo.
Normally, a received raw signal is not used for evaluation, rather, from the raw signal, an echo-curve is derived, which is then evaluated. In the Pulse-Radar method, the echo-curve is e.g. a hull, or envelope, curve derived from the raw signal. The hull curve is established by rectifying and filtering the raw signal. In the Frequency Modulated Continuous Wave method, the echo-curve is e.g. a frequency spectrum derived from the raw signal. For the exact determination of a travel time of the usable echo, first a maximum of the echo curve is determined.
Fill level measurement according to the travel time principle depends only on the constant propagation velocity of the signals, and thus supplies very accurate results.
However, in fill level measurement according to the travel time principle, difficulties exist when, for example, multiple echoes are present due to fixtures in the container or due to multiple reflections of the signals.
Multiple reflections occur e.g. when a send-pulse is reflected back and forth repeatedly between the upper surface of the fill substance and a lid of the container.
The results of the measurements are thus very accurate as long as the correct echo is recognized. However, if the correct echo is not recognized, free of doubt, then a very great uncertainty of the measurements will arise. Plausible assumptions concerning the maximum filling- and emptying velocities or concerning the relative strengths of the signals can in fact, in some cases, ensure the selection of the correct echo; however, due to the wide variety of various measuring situations, all possible cases cannot be covered therewith.